The paramyxovirus respiratory syncytial (RS) virus is the major viral cause of bronchiolitis and pneumonia in infants and children and no effective vaccine is available. The situation is compounded by the fact that maternal antibody does not confer solid immunity on neonates and natural infection affords only partial protection against frequent repeat infections.To address this major public health problem, Dr. Wertz's research program over the last 10 years has focused on the molecular biology of RS virus, characterizing the viral genes in some detail, and examining the individual immune responses to 9 of the 10 viral gene products. But progress, though substantial, has been hindered by the fact that it has not been possible to engineer mutations into the genomes of non-segmented negative strand RNA viruses such as RS virus, and to recover infectious mutants. However, a recent breakthrough in the work on vesicular stomatitis virus (VSV) has changed this situation. Dr. Wertz has developed a method to recover infectious VSV defective interfering particles from cDNA clones. The method has general applicability, so for the first time it appears feasible to construct a full-length cDNA clone of the RS virus genome, and from it to recover infectious virus. Plans to pursue this goal are presented in this proposal, together with the initial ways that the infectious clone will be used to address outstanding problems in RS virus biology and pathogenesis. The technology developed with VSV will be applied to RS virus to carry out reverse genetic analysis of the cis- and trans-acting factors involved in RS virus RNA replication, nucleocapsid assembly and budding of infectious particles, and to generate a replicable full-length genomic RNA from a cDNA clone in order to address crucial questions in the biology of this pathogen.